1. Field of the Invention
The present invention relates to a two-phase brushless direct-current (BLDC) motor having a single Hall effect device, and more particularly, to a BLDC motor having a single position sensing device in which a driving circuit is very simple since a rotor can be driven without creating a dead point and a high efficiency and a small torque ripple are accomplished since magnets are efficiently arranged.
2. Description of the Related Art
Generally, BLDC motors are classified into a cup-shaped (cylindrical) core type (or a radial type) and a coreless type (or an axial type), depending upon whether or not a stator core exists.
The core type BLDC motors are classified into an internal magnet type BLDC motor including a cylindrical stator in which coils are wound on a plurality of protrusions formed on its inner circumference to provide electromagnetic structure and a rotor composed of cylindrical permanent magnets and an external magnet type BLDC motor including a stator in which coils are wound up and down on a plurality of protrusions formed on its outer circumference and a rotor composed of multipolar magnetized cylindrical permanent magnets outside the stator.
Since its magnetic circuit has an axially symmetrical structure in radial direction, the core type BLDC motor makes little vibratile noise during operation and is suitable for low-speed rotation, providing an excellent torque. However, the core type BLDC motor results in many a loss of material for a yoke during fabrication of its stator and requires great expense for facility investment for mass production. In addition, since its stator and rotor are of complicated structure, it is not easy to make the motor compact, and it cannot assure high efficiency and creates an undesirable cogging torque.
Meanwhile, a conventional coreless BLDC motor provided in order to improve the disadvantages of the above-described core type BLDC motor has such a structure that rotors each of which consists of an annular magnet and a yoke are fixed to a rotational shaft and stators each formed by winding a plurality of stator coils on a printed circuit board (PCB) are connected to the rotational shaft via bearings.
In the coreless BLDC motor, a magnetic circuit is axially formed between the rotors incorporated with a plurality of sets of magnets and stators composed of stator coils generating a plurality of sets of electromagnetic force which are disposed below the rotors. Thus, although a buffer spring is inserted between the bearings, the coreless BLDC motor generates great axial vibrations due to the stators' attracting or repelling force and their unequal magnetization.
Besides, the axial vibrations induces a resonance of the overall system employing the coreless BLDC motor, thereby increasing noise during rotation. Accordingly, the entire motor's efficiency is excellent without having a great loss during high-speed rotation, but may induces abnormal noise since the rotational noise is mixed with the vibratile noise.
As a result, the coreless BLDC motor can minimize a loss of materials, having a good mass productivity and making the motor thinner and more compact, thereby accomplishing low cost and high efficiency in comparison to the core BLDC motor, but much noise is generated due to the axial vibration during rotation.
To solve the above demerits of the coreless BLDC motor, the same applicant has proposed a double rotor-double stator coreless BLDC motor which can offset the axial vibration generated during rotation of the rotor and increase the torque more than two times. Since the motor has the double-double stator structure in which a plurality of wound stator coils are mounted in the middle of respective first and second double rotors on the left and right sides of the PCB, a magnetic circuit is formed in symmetry with the whole stators' and rotors' rotational shaft. Thus, the double rotor/double stator structure offsets the attracting or repelling force acting on the first and second rotors by the stators, thereby minimizing the axial vibrations of the rotors.
Meanwhile, the same applicant has proposed an improved invention as shown in FIG. 1, in which the stators of the double rotor/double stator coreless BLDC motor is incorporated into a single body by an insert molding, thereby increasing durability and saving production cost. Referring to FIG. 1, the BLDC motor will be described below.
In the conventional BLDC motor as shown in FIG. 1, an outer circumference 67 of a stator 51 is extended up and down in the middle of upper and lower cases 71A and 71B, to thereby form a cylindrical casing.
An upper rotor 73A and a lower rotor 73B having a magnet division multipolar arrangement structure spaced by a predetermined air gap from the upper and lower portion of the stator 51 are fixedly coupled to a rotational shaft 77 via bushings 75A and 75B at the center of the rotational shaft 77.
In the case that the BLDC motor is a three-phase driven motor, each rotor 73A or 73B has such a structure that a magnetic circuit is formed with respect to twelve magnets 81A and 81B, in which the twelve magnets 81A and 81B, that is, six disc-shaped N-pole magnets 81A and six disc-shaped S-pole magnets 81B are alternately formed as non-ferromagnetic materials such as PET (polyethylene terephthalate), nylon-66 or PBT (polybutylene terephthalate). Here, a circumference of each rotor is supported by a support 79 integrally formed on the bushings 75A and 75B, and annular magnetic yokes 83A and 83B are attached to the rear surfaces of the rotors.
Meanwhile, a position detection auxiliary magnet 85 is disposed at the upper side of the magnetic yoke 83A in the upper rotor 73A. The auxiliary magnet 85 is disposed in correspondence to three Hall effect devices (H1-H3) 89 in a control PCB 87 fixedly disposed in the inner circumference of the upper case 71A. A female connector 91 to which the upper terminal 63 of the stator 51 is forcedly connected is mounted on one side of the control PCB 87.
Upper and lower bearings 93A and 93B are fixedly mounted on the central portions of the upper case 71A and the lower case 71B. The rotational shaft 77 of the rotors 73A and 73B is rotatably supported through the bearings 93A and 93B. Also, the stator 51 is comprised of nine angular bobbin coils or bobbinless coils (L1-L3) 55 which are formed in the form of a disc by an insert molding method using a resin insulating material, on an auxiliary PCB 57.
In the conventional three-phase driven BLDC motor, the rotor is comprised of twelve N-pole and S-pole magnets 81A and 81B which are disposed at an equal interval, and the stator is comprised of nine coils 55 which are disposed at an equal interval, both of which are shown in FIG. 2.
In the stator, nine coils 55 are divided into three phases, e. g. each phase u, v, w including three coils which are connected in series and connected in the star (Y) connection as shown in FIG. 4. When positions of the rotors are detected sequentially by the three position detection Hall effect devices H1-H3, a switching transistor 97 is so driven that current sequentially flows through coils of two phases among the stator coils L1-L3 of the three phases, at a predetermined angle by a three-phase logic integrated circuit (IC) 95. In this case, three-phase induced electromotive force curves u, v and w each formed as a sinusoidal wave in the conventional motor are shown in FIG. 3.
That is, the end points of the three phases are connected to each other in the BLDC motor employing the three-phase full-wave driving method. In view of one phase, three processes are repeated in such a manner that current flows in one direction, flows in the opposite direction and then sinks.
Thus, the three-phase full-wave driving method requires three Hall effect devices, three-phase logic IC and six switching transistors, which are considerably expensive in their product cost.
Meanwhile, a general two-phase full-wave driving method requires two Hall effect devices and four switching transistors and a general two-phase half-wave driving method requires two Hall effect devices and two switching transistors. However, a non-start zone or dead point essentially exists in the two-phase half-wave driving system. Thus, a special counter-measure is needed in order to avoid the non-start zone or the dead point at the start of the motor. Also, since an induced electromotive force is low, the efficiency of the motor is low and a great torque ripple appears.
As a prior art reference, U. S. Pat. No. 4,211,963 discloses a two-phase driven BLDC motor including a non-symmetrical rotor subdivided into two monopole zones having uniform magnetization throughout their radial extent and two dipole zones of magnetization having opposite poles of magnetization with respect to their radial extent, and a single Hall effect device for detecting position of the rotor.
Since the two dipole zones used for the positional detection of the rotor are subdividingly magnetized and arranged on the same circumference of a single permanent magnet together with the two monopole zones in U. S. Pat. No. 4,211,963, magnetization of the permanent magnet forming the rotor is in trouble.